The present invention relates to an electromagnetic flowmeter and, more particularly, to an electromagnetic flowmeter having a function of switching/controlling the current value of an exciting current in each range of flow rates that have been measured.
An electromagnetic flowmeter supplies an exciting current to a coil to apply a magnetic field to a fluid in a pipe line, and measures the flow rate of the fluid on the basis of the signal electromotive force detected from electrodes disposed in the pipe line. In this electromagnetic flowmeter, various kinds of noise are produced, e.g., commercial power noise mixed in the fluid, spike noise produced by the collision of foreign substances contained in the fluid with the electrodes, electrochemical noise caused by charge ions in the fluid, and noise dependent on the flow velocity and conductivity. In order to reduce the influences of such noise, the current value of an exciting current may be increased to increase the S/N ratio.
In some electromagnetic flowmeters, however, the current value of an exciting current that can be used is limited. For example, in a two-wire electromagnetic flowmeter designed to supply power and transmit a flow rate signal through a pair of signal lines, an obtained measurement flow rate is converted into a value from 0 to 100% in predetermined range, which in turn is converted into a flow rate signal formed from a loop current value from 4 to 20 mA which corresponds to the value from 0 to 100%, and the signal is output. Since the two-wire electromagnetic flowmeter uses this loop current as operating power, an exciting current of minimum 4 mA must be supplied, and the overall two-wire electromagnetic flowmeter must be operated.
Conventionally, as such a two-wire electromagnetic flowmeter, an electromagnetic flowmeter has been proposed (e.g., Japanese Patent Laid-Open No. 8-50043), in which if there is enough operating power, i.e., loop current, the current value of an exciting current is increased to increase the S/N ratio. In this two-wire electromagnetic flowmeter, as shown in FIG. 9, measured flow rates (% value) are classified into three flow rate ranges I to III, and current values of 4 mA, 8 mA, and 12 mA are used as exciting currents Iex in the respective flow rate ranges I, II, and III. This makes it possible to perform measurement by using large current values in the flow rate ranges II and III to obtain high S/N ratios as compare with the case wherein an exciting current of 4 mA is always used.
In such a conventional electromagnetic flowmeter, however, since the current value of an exciting current is switched/controlled in each flow rate range, a measurement error occurs with a change in the current value of an exciting current. For example, as shown in FIG. 10, when an AC exciting current in the form of a rectangular wave is applied to the coil, the waveform of a magnetic flux density B produced by the exciting current Iex that actually flows suffers a delay in accordance with the characteristics of the coil. As a result, magnetic flux differential noise is produced in the signal electromotive force obtained from the detection elements, i.e., the AC flow rate signal.
In general, in order to suppress the influences of such magnetic flux differential noise, a sampling period during which an AC flow rate signal is sampled is set in the trailing edge portion of the pulse-like waveform which is affected little by the magnetic flux differential noise. If, however, the current value of the exciting current is increased, the magnetic flux differential noise also increases (see the broken lines in FIG. 10), and a voltage difference Δe occurs in the sampling period even at zero flow rate. This difference appears as an error in a measurement flow rate.
In addition, according to the electromagnetic flowmeter, when a measurement flow rate is calculated by using the signal electromotive force obtained by the detection electrodes, a flow rate v is obtained by e=k·B·v·D where e is a signal electromotive force, k is a constant, B is a magnetic flux, and D is the diameter of the pipe line. In this case, the magnetic flux density B is approximated by the exciting current Iex assuming that the magnetic flux density B is proportional to a magnetic field H, and the magnetic field H is proportional to the exciting current.
In general, a coil has a nonlinear characteristic as a magnetic field-magnetic flux density characteristic. Even if, therefore, the magnetic field-magnetic flux density characteristic is linearly approximated, no large error is generated unless the magnetic field H excessively changes. For example, as shown in FIG. 11, with a magnetic field H1 and magnetic flux density B1, even if the magnetic flux density in a range equal to or less than the magnetic field H1 is approximated by B=a1·H1 (for a1=B1/H1), no large error is generated.
If, however, the magnetic field changes greatly as a large exciting current is used by extending the exciting current switching range, the magnetic field-magnetic flux density characteristic cannot be linearly approximated, resulting in a large error. For example, with a magnetic field H2 and magnetic flux density B2, if the characteristic is linearly approximated in the same manner as described above, the magnetic flux density at the magnetic field H2 or less is given by B=a2·H2 (for a2=B2/H2). Consequently, an error ΔB occurs between approximated magnetic flux density B′=a2·H1 and an actual magnetic flux density B1, resulting in an error in a measurement flow rate.